Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. By reducing displacement under low torque request situations, the engine is operated at a higher manifold pressure, reducing engine friction due to pumping, and resulting in reduced fuel consumption.
As such, abnormal combustion events, such as those due to pre-ignition can occur in a VDE engine. One example approach for addressing pre-ignition events occurring in a VDE engine system is shown by Kerns et al. in US 20120285161. Therein, a threshold and window for pre-ignition detection is adjusted during a VDE mode of operation based on a number of deactivated cylinders. The threshold is also varied between VDE and non-VDE modes to better compensate for background noise differences, thereby improving pre-ignition detection during VDE and non-VDE modes.
However, the inventors herein have identified potential issues with such an approach. As an example, during selected cylinder reactivations, pre-ignition may be induced. Thus, even if the pre-ignition is detected accurately in the VDE mode and addressed, pre-ignition may continue to occur when the engine is transitioned to the non-VDE mode. In other words, during selected conditions, such as when operating with one or more cylinders deactivated for a significant amount of time, a likelihood of abnormal combustion, such as due to cylinder pre-ignition, may increase. This is due to the accumulation of oil in the deactivated engine cylinders. For example, during long steady-state highway cruising conditions, the deactivated cylinders may collect a fair amount of oil because of vacuum created in the deactivated engine cylinders due to continued engine spinning. Oil may also be drawn in due to lower temperatures in the cylinder during deactivation operation, as well as lower pressures on the oil control ring of the piston. As such, the lower temperatures and pressures allow oil to migrate into the combustion chamber, and collect therein. The trapped oil can then act as an ignition source during subsequent cylinder reactivation. In some engine systems, control strategies may be applied to cylinders after extended operation in deactivation mode to help restore pressure on the oil control rings. However, in a boosted engine, if one or more cylinders are deactivated for an extended period, and this is followed by a significant increase in torque demand (such as during a passing maneuver) where boost is maintained or increased and the cylinders are reactivated, the oil trapped in the cylinder(s) may become an ignition source leading to pre-ignition events, poor NVH (audible knocking) and potential engine damage. In particular, the combustion of the trapped oil may cause high in-cylinder pressures and temperatures associated with pre-ignition that can degrade engine components as well as decrease engine efficiency.
In one example, some of the above issues may be at least partly addressed by a method for an engine comprising: while reactivating a cylinder to a higher than threshold load condition, and before an indication of pre-ignition in the cylinder is received, enriching the reactivated cylinder, the enrichment adjusted based on each of cylinder load and a preceding duration of cylinder deactivation. In this way, pre-ignition occurring during cylinder reactivation to high loads following prolonged cylinder deactivation may be reduced.
For example, an engine may be configured with selectively deactivatable cylinder fuel injectors and/or valves. During conditions of low torque demand, one or more engine cylinders may be selectively deactivated and the torque demand may be met via the remaining active cylinders. In response to a subsequent increase in operator torque demand, the cylinders may be reactivated. As such, due to engine operation during the deactivation of the selected cylinders, oil may accumulate in the deactivated cylinders, which may ignite if the cylinder load is too high. Therefore, if the increase in operator torque demand is substantially high, and the cylinder load of the reactivated cylinders exceeds a threshold, the reactivated cylinders may be operated richer than stoichiometry for a duration to mitigate potential pre-ignition caused by combustion of the accumulated oil. Herein, the enrichment may be performed preemptively, before an actual indication of pre-ignition is received. A degree of cylinder enrichment may be adjusted based on the duration for which the cylinder was previously deactivated as well as the load in the cylinder upon reactivation. As such, more oil may accumulate in the deactivated cylinder as the duration increases. Likewise, the propensity for cylinder pre-ignition may increase as the cylinder load upon reactivation increases. Thus, a degree of richness and a number of enrichment cycles may be increased as the duration of deactivation and the cylinder load increases. If no pre-ignition occurs during the reactivation, on a subsequent reactivation to high load, an enrichment of the given cylinder may be trimmed. Alternatively, if pre-ignition does occur during the reactivation, on a subsequent reactivation to high load, an enrichment of the given cylinder may be increased. As such, the pre-emptive enrichment may not be performed in cylinders that were deactivated as part of an engine shut-down operation, such as during an idle-stop operation or a deceleration fuel shut-off operation since significant oil accumulation does not occur during such deactivations.
In this way, pre-ignition propensity in a cylinder being reactivated to high loads from a deactivated condition can be reduced. By preemptively enriching a cylinder that was selectively deactivated while the engine continued to spin, pre-ignition resulting from the combustion of oil that accumulated in the cylinder during the deactivation can be better anticipated and addressed. In addition, the enrichment provides cylinder cooling which further reduces pre-ignition events in the cylinder during reactivation to high loads. By adjusting the enrichment in a closed-loop fashion based on the occurrence of pre-ignition events during the reactivation, the enrichment can be optimized, reducing fuel wastage and emissions output. Overall, cylinder pre-ignition can be better addressed in a variable displacement engine during reactivation to high loads.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.